docs: def expr(self, callback):
          """Filter by custom expression.
  
          :param  callback: Callback for custom expression.
          :type   name: ``function``
  
          :return: A list of Node objects.
          :rtype:  ``list`` of :class:`Node`
          """
          nodes = []
          for node in self.nodes:
              if callback(node):
                  nodes.append(node)
          self.nodes = nodes
          return self